In a long optical fiber path, optical amplifiers at various intervals restore optical signal levels. Optical amplification restores the signal level, but does not correct chromatic dispersion that may accumulate along the optical path. Chromatic dispersion compensating elements can also be installed in conjunction with the amplifiers, so that all signal characteristics are restored.
The actual fiber length between amplification sites varies widely, as does the dispersion per unit length for different spools of fiber. This makes dispersion compensation a field-determined value. A straight-forward means of accomplishing the dispersion compensation is to measure the dispersion characteristic of each section of fiber (between amplification/compensation sites or nodes) and then to install the appropriate amount of dispersion compensation at the end of each section. One particularly attractive scheme of dispersion compensation uses a special compensating optical fiber that has dispersion characteristics complementary to those of the main fiber path. In this method, the amount of compensation is proportional to the length of the compensating fiber. This method for dispersion compensating, therefore, suffers from significant limitations.
Since it is impractical to adjust the length of compensating fiber on-location in the field, this method requires that large numbers of different lengths of compensating fibers be readily available. The large inventory of different lengths makes this method difficult to employ in the field.
Consequently, it is object of the present invention to provide a method for compensating each optical fiber section in a low-resolution manner so that only a small number of different lengths of compensating fiber are necessary. The present invention eliminates dispersion compensation error build-up by measuring the net dispersion from the transmit end of the path to each compensating site. It is the object of the present invention, therefore, to provide over-compensation and under-compensation at various nodes along the optical path with a view to resulting in a final end-to-end dispersion of the entire optical path within acceptable limits.
It is another object of the present invention to provide a method for compensating end-to-end optical dispersion for a fiber-optic cable having a plurality of predetermined compensation sites to within a predetermined dispersion limit by using a predetermined dispersion compensating increment and compensating optical dispersion within the compensation increment to the closest compensating point available provided that the total end-to-end optical dispersion is kept within the predetermined limit. This includes the possibility for over-compensating at some sites while under-compensating at others. By interspersing the over-compensation and under-compensation, the method yields an end-to-end optical dispersion compensation that is within the predetermined optical dispersion limit.